1. Field of the Invention
The present invention relates to an electronic device comprising a mount substrate and an electronic component mounted on the mount substrate, and to a method of manufacturing such an electronic device.
2. Description of the Related Art
Surface acoustic wave devices include interdigitated electrodes formed on a surface of a piezoelectric substrate. Surface acoustic wave devices are widely used as filters in mobile communication equipment such as cellular phones.
An electronic component such as a semiconductor component is frequently used in the form of a package. The package is configured such that the electronic component is mounted on a mount substrate, connection electrodes of the electronic component are electrically connected to patterned conductors of the mount substrate, and portions connecting the connection electrodes of the electronic component and the patterned conductors of the mount substrate are sealed. The same holds true for the case where the electronic component is a surface acoustic wave device. In the present application, a device which comprises a mount substrate and an electronic component mounted on the mount substrate is referred to as an electronic device.
For electronic devices, methods for electrically connecting the connection electrodes of the electronic component to the patterned conductors of the mount substrate are broadly divided into two: a face-down bonding and a face-up bonding. In the face-down bonding, the electronic component is placed such that the surface of the electronic component having the connection electrodes formed thereon faces toward the mount substrate. On the other hand, in the face-up bonding, the electronic component is placed such that the surface of the electronic component having the connection electrodes formed thereon, faces away from the mount substrate. The face-down bonding is advantageous over the face-up bonding in terms of downsizing of the electronic device.
In general, according to a conventional method of manufacturing an electronic device employing the face-down bonding, after electrically connecting the connection electrodes of the electronic component and the patterned conductors of the mount substrate, an underfill material is filled in between the electronic component and the mount substrate to seal the portions where the connection electrode of the electronic component and the patterned conductor of the mount substrate are electrically connected to each other.
When a surface acoustic wave device is used as the electronic component, however, the aforementioned general method cannot be employed because of a problem typical of the surface acoustic wave device. The problem typical of the surface acoustic wave device derives from the fact that the surface acoustic wave device has interdigitated electrodes formed on a surface thereof. That is, while the surface acoustic wave device needs to be sealed in order to prevent the interdigitated electrodes from being contaminated by moisture or foreign materials such as dust particles, it is necessary to prevent a resin or the like for the sealing from touching the surface acoustic wave propagation region on the surface of the surface acoustic wave device so as not to adversely affect the operation of the surface acoustic wave device.
Therefore, in a conventional method for manufacturing an electronic device comprising a surface acoustic wave device as its electronic component, after the connection electrode of the surface acoustic wave device is electrically connected to the patterned conductor of the mount substrate, the surface acoustic wave device is enclosed and sealed with a cap-like structure formed of ceramics or metal. There is another method available for manufacturing an electronic device comprising a surface acoustic wave device as its electronic component. In this method, after the connection electrode of the surface acoustic wave device is electrically connected to the patterned conductor of the mount substrate, the surface acoustic wave device is enclosed and sealed with a side-fill material.
In each of Published Unexamined Japanese Patent Applications Heisei 10-125825, 2001-53092, and 2001-176995, disclosed is a method of manufacturing an electronic device comprising a chip-type electronic component mounted on a substrate.
According to the method disclosed in Published Unexamined Japanese Patent Application Heisei 10-125825, the chip-type electronic component is first mounted on the substrate through flip chip bonding. Then, the electronic component is covered with a sealing film made of a resin. Then, the film is pressed against the substrate at around the electronic component with a stamping die to adhere the film to the substrate, thereby sealing the electronic component with the film. In Published Unexamined Japanese Patent Application Heisei 10-125825, disclosed are two methods for adhering the sealing film to the substrate: one making use of adhesiveness of the film and the other employing an adhesive.
In the method disclosed in Published Unexamined Japanese Patent Application 2001-53092, the chip-type electronic component is first mounted on the substrate. Then, an adhesive is applied to areas of a flexible sheet and/or the substrate at which the sheet and the substrate are to be bonded to each other. The electronic component is then covered with the sheet. Then, a gas captured in a cavity defined by the sheet and the substrate is sucked through a small hole provided in the substrate. Thereafter, the sheet is pressed against the substrate using a shaping tool, thereby causing the adhesive to bond the sheet to the substrate.
In the method disclosed in Published Unexamined Japanese Patent Application 2001-176995, the chip-type electronic component is first mounted on the substrate. Then, a deformed film is placed onto the electronic component. A gas captured therebetween is sucked through a hole provided in the substrate to fit the deformed film onto the electronic component and adhere the deformed film to the substrate. In Published Unexamined Japanese Patent Application 2001-176995, disclosed are two methods for adhering the deformed film to the substrate: one employing an adhesive and the other making use of the thermo-adhering property of the deformed film.
Among the methods of manufacturing the electronic device comprising a surface acoustic wave device as its electronic component, the one in which the surface acoustic wave device is enclosed and sealed with a cap-like structure presents a problem in that it is difficult to downsize the electronic device. Another problem with this method is that it is impossible to improve the strength and stability of mechanical bonding between the connection electrode of the surface acoustic wave device and the patterned conductor of the mount substrate, because the aforementioned structure does not contribute to the mechanical bonding between the connection electrode of the surface acoustic wave device and the patterned conductor of the mount substrate.
Among the methods of manufacturing the electronic device comprising a surface acoustic wave device as its electronic component, the one in which the surface acoustic wave device is enclosed and sealed with a side-fill material has a problem in that the side-fill material can enter the surface acoustic wave propagation region on the surface of the surface acoustic wave device.
When the electronic device is an oscillator or a high-frequency circuit component, too, as in the case of the surface acoustic wave device, operation of the electronic component can be adversely affected by a sealing resin if the resin touches the surface of the electronic device. Accordingly, the aforementioned problems also arise when an oscillator or a high-frequency circuit component is used as the electronic component, as in the case of a surface acoustic wave device.
On the other hand, according to the methods disclosed in Published Unexamined Japanese Patent Applications Heisei 10-125825, 2001-53092, and 2001-176995, it is possible to seal the electronic component without causing the sealing resin or the like to touch the surface of the electronic component.
The method disclosed in Published Unexamined Japanese Patent Application Heisei 10-125825, however, involves mechanical driving of the stamping die, which requires large-scale equipment to manufacture the electronic device. The manufacturing process therefore becomes complicated.
The method disclosed in Published Unexamined Japanese Patent Application 2001-53092 involves a cumbersome step of applying an adhesive to the areas of the sheet and/or the substrate at which the sheet and the substrate are to be bonded to each other.
According to the method disclosed in Published Unexamined Japanese Patent Application 2001-176995, the deformed film is used to package the electronic component. In this publication, disclosed is a technique of fitting the deformed film onto the electronic component by suction through a hole provided on the substrate. However, the publication does not disclose that the film is deformed by the suction through the hole provided in the substrate. It is thus conceivable that the deformed film mentioned in this publication is a film that has been shaped in advance to fit the outer shape of the electronic component. Therefore, the method disclosed in Published Unexamined Japanese Patent Application 2001-176995 requires the step of preparing the deformed film, and this step is also cumbersome.
For a band pass filter employing a surface acoustic wave device, the center frequency of its passband is set, for example, at around 1800 MHz. In this case, a permissible shift of the center frequency of an actual filter from the desired center frequency is about +/−1.2 MHz at maximum. The band pass filter employing a surface acoustic wave device thus requires its center frequency to be set with extremely high accuracy.
On the other hand, in the case of manufacturing the electronic device through the steps of covering the surface acoustic wave device mounted on the substrate with a resin film, and then heating the film to thereby adhere the film to the substrate, there is a problem in that the processing for adhering the film may cause a change in characteristics of the surface acoustic wave device. This is conceivably because at the initial stage of heating the film at a high temperature, the volatile components in the resin of the film come out into the environment and deposit on the surface acoustic wave device. For a band pass filter employing a surface acoustic wave device, a change in the characteristics of the surface acoustic wave device can cause a shift of the center frequency in a passband of the filter, and a degradation in transmission characteristics of the filter such as an increase in the insertion loss, in particular.